"In geometry, a protractor is a square, circular or semicircular tool, typically in transparent perspex, for measuring angles. The units of measurement utilized are usually degrees (°).
"They are used for a variety of mechanical and engineering-related applications, but perhaps the most common use is in geometry lessons in schools.
"Some protractors are simple half-discs. More advanced protractors, such as the bevel protractor, have one or two swinging arms, which can be used to help measure the angle." http://en.wikipedia.org/wiki/Protractor